Particulate matter treatment system

ABSTRACT

A particulate matter treatment system includes an electrode provided in an exhaust passage of an internal combustion engine, a power supply connected to the electrode and operable to apply a voltage to the electrode, a particle number detector that detects the number of particles of particulate matter downstream of the electrode, and a determining device that determines that the system is at fault when an absolute value of the amount of change in the number of particles of particulate matter detected by the particle number detector when the voltage applied from the power supply to the electrode is changed is smaller than a threshold value.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application No. 2011-087257 filed on Apr. 11, 2011, which is incorporated herein by reference in its entirety including the specification, drawings and abstract.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a particulate matter treatment system.

2. Description of Related Art

For example, Japanese Patent Application Publication No. 2006-194116 (JP-A-2006-194116) discloses a technology of providing a discharge electrode in an exhaust passage of an internal combustion engine, and charging particulate matter (which will also be called “PM”) by allowing the discharge electrode to generate corona discharge, so as to aggregate the PM. With the PM thus aggregated, the number of particles of the PM can be reduced. Also, the particle size of the PM is increased, so that the PM can be more easily trapped by a filter provided downstream of the electrode.

In some cases, the particulate matter treatment system fails or deteriorates, which makes it difficult to aggregate the PM. Accordingly, it is also important to detect a failure of the system.

SUMMARY OF THE INVENTION

The invention provides a particulate matter treatment system that detects a failure of the system.

A particulate matter treatment system according to one aspect of the invention includes an electrode provided in an exhaust passage of an internal combustion engine, a power supply connected to the electrode and operable to apply a voltage to the electrode, a particle number detector that detects the number of particles of particulate matter downstream of the electrode, and a determining device that determines that the particulate matter treatment system is at fault when an absolute value of an amount of change in the number of particles of particulate matter detected by the particle number detector when the voltage applied from the power supply to the electrode is changed is smaller than a threshold value.

When a voltage is applied to the electrode, the PM can be charged. The charged PM moves toward the inner wall of the exhaust passage under coulomb force and flow of exhaust gas. The PM that has reached the inner wall of the exhaust passage releases electrons to the exhaust passage, so that electricity flows to the ground side rather than the electrode. Then, the PM particles that have released electrons aggregate or clump together with other PM particles present in the vicinity of the above PM, so that the number of particles can be reduced.

The number of particles of particulate matter detected by the particle number detector when a voltage is applied to the electrode is the number of particles detected after aggregation of the PM. If the voltage applied to the electrode is increased, a larger number of electrons are released from the electrode. As a result, the aggregation of the PM can be promoted, and the number of PM particles can be further reduced. Namely, when the particulate matter treatment system is normal, the number of particles of particulate matter detected by the particle number detector changes according to the voltage applied to the electrode. Accordingly, when the voltage applied to the electrode is changed, the number of PM particles detected by the particle number detector should change if the particulate matter treatment system is normal. The absolute value of the amount of change in the number of PM particles responsive to a change of the applied voltage is relatively large if the particulate matter treatment system is normal, and is relatively small if the system is at fault. Namely, even if an attempt to change the voltage applied to the electrode is made, the voltage may not actually change, or the change of the voltage may be insufficient when the particulate matter treatment system is at fault. For example, when no voltage is applied to the electrode due to a failure, the voltage does not actually change even if an attempt to change the voltage applied to the electrode is made, and the amount of change in the number of PM particles is equal to zero.

Thus, if the particulate matter treatment system is at fault, the aggregated PM is reduced. As a result, the absolute value of the amount of change in the number of particles of particulate matter detected by the particle number detector when the voltage is changed becomes relatively small. Accordingly, it can be determined that the system is at fault when the absolute value of the amount of change in the number of PM particles is smaller than the threshold value. The threshold value may be set to a lower limit value of the absolute value of the amount of change in the number of particles when the particulate matter treatment system is normal. It may be determined that the particulate matter treatment system is at fault when the number of PM particles detected by the particle number detector does not change before and after the applied voltage is changed.

When a failure detection is performed, the amount of change in the number of particles can be promptly detected by positively changing the voltage applied to the electrode; therefore, the process of making a failure decision can be promptly completed. Also, a failure decision may be made based on an absolute value of the amount of change in the number of particles while the applied voltage is varied within a preset range.

With the voltage thus changed, the number of particles detected by the particle number detector may increase or may decrease. The amount of change is a positive value when the number of particles increases, and is a negative value when the number of particles decreases. Therefore, the absolute value of the amount of change is used. It may also be determined that the particulate matter treatment system is at fault when the amount of change in the number of particles is within a specified range that ranges from a negative value to a positive value.

According to the above aspect of the invention, a failure of the particulate matter treatment system can be detected.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 is a view schematically showing the construction of a particulate matter treatment system according to a first embodiment of the invention;

FIG. 2 is a flowchart illustrating the control flow of failure determination according to the first embodiment;

FIG. 3 is a view concerned with a second embodiment of the invention, showing the relationships among the applied voltage, the number of PM particles, and the percentage of reduction of the number of PM particles; and

FIG. 4 is a view concerned with the second embodiment of the invention, showing the relationships among the applied voltage, the number of PM particles, and the percentage of reduction of the number of PM particles.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 schematically shows the construction of a particulate matter treatment system 1 according to a first embodiment of the invention. The particulate matter treatment system 1 is provided in an exhaust passage 2 of a gasoline engine, for example. The particulate matter treatment system 1 may also be provided in an exhaust passage of a diesel engine.

The particulate matter treatment system 1 includes a housing 3 connected at opposite ends to an exhaust passage 2. The housing 3 is formed of stainless steel. The housing 3 is formed in the shape of a circular cylinder having a larger diameter than that of the exhaust passage 2. The opposite end portions of the housing 3 are tapered such that the cross-sectional area of the housing 3 decreases toward the opposite ends thereof. In operation, exhaust gas flows in the direction of the arrow indicated in FIG. 1, and flows into the housing 3. Thus, the housing 3 may be regarded as a part of the exhaust passage 2.

The exhaust passage 2 and the housing 3 are connected to each other via insulating portions 4. Each of the insulating portions 4 is sandwiched by and between a flange 21 formed on an end portion of the exhaust passage 2, and a flange 31 formed on an end portion of the housing 3. The exhaust passage 2 and the housing 3 are fastened to each other with, for example, bolts and nuts. The bolts and nuts are also subjected to electric insulating treatment, so that electricity does not flow through the bolts and nuts. With this arrangement, no electricity flows between the exhaust passage 2 and the housing 3.

An electrode 5 is mounted in the housing 3. The electrode 5 penetrates a side face of the housing 3, and extends from the side face of the housing 3 toward the center axis of the housing 3. Then, the electrode 5 bends in a direction opposite to the direction of flow of exhaust gas at around the center axis, and extends toward the upstream side of the flow of exhaust gas, in parallel with the center axis. Thus, an end portion of the electrode 5 is located in the vicinity of the center axis of the housing 3. The electrode 5 is provided with an insulator portion 51 consisting of an electric insulator, so that no electricity flows between the electrode 5 and the housing 3. The insulator portion 51 is located between the electrode 5 and the housing 3, and has the functions of insulating electricity, and also fixing the electrode 5 to the housing 3.

The electrode 5 is connected to a power supply 6 via a power-supply-side electric wire 52. The power supply 6 applies current to the electrode 5, and is able to change the applied voltage. The power supply 6 is connected to a controller 7 and a battery 8 via electric wires. The controller 7 controls the voltage applied from the power supply 6 to the electrode 5.

Also a ground-side electric wire 53 is connected to the housing 3, and the housing 3 is grounded via the ground-side electric wire 53. The ground-side electric wire 53 is provided with a detector 9 that detects current passing through the ground-side electric wire 53. The detector 9 detects the current by measuring a potential difference between opposite ends of a resistor provided at some midpoint in the ground-side electric wire 53. The detector 9 is connected to the controller 7 via an electric wire, so that the controller 7 receives the current detected by the detector 9. The power-supply-side electric wire 52 may be provided with a detector, like the detector 9, for detecting current passing through the power-supply-side electric wire 52.

A particle number sensor 75 that detects the number of particles of PM (particulate matter) in exhaust gas is provided in the exhaust passage 2 downstream of the housing 3. The particle number sensor 75 detects the number of particles of PM per unit volume in the exhaust gas. The particle number sensor 75 is connected to the controller 7 via an electric wire, so that the controller 7 receives the number of particles of PM detected by the particle number sensor 75. In this embodiment, the particle number sensor 75 corresponds to the above-indicated particle number detector of the invention.

An acceleration stroke sensor 71, a crank position sensor 72, a temperature sensor 73, and an airflow meter 74 are connected to the controller 7. The acceleration stroke sensor 71 generates an electric signal representing the amount of depression of the accelerator pedal by the driver of the vehicle in which the internal combustion engine is installed, so as to determine the engine load. The crank position sensor 72 detects the engine speed. The temperature sensor 73 detects the temperature of a coolant or a lubricating oil of the internal combustion engine, so as to determine the temperature of the internal combustion engine. The airflow meter 74 detects the amount of intake air of the internal combustion engine.

In the particulate matter treatment system 1 constructed as described above, a negative DC high voltage is applied from the power supply 6 to the electrode 5, so that electrons are released from the electrode 5. Namely, the potential of the electrode 5 is made lower than that of the housing 3, so that electrons are released from the electrode 5. Then, the electrons thus released can negatively charge PM contained in the exhaust gas. The negatively-charged PM moves under the influences of coulomb force and gas flow. Then, when the PM reaches the housing 3, the electrons that negatively charged the PM are released to the housing 3. The PM particles that have released electrons to the housing 3 aggregate or clump together so that the particle size of the PM is increased. With the PM thus aggregated, the number of particles of PM is reduced. Namely, it is possible to increase the particle size of the PM and reduce the number of particles of the PM, by applying a voltage to the electrode 5.

While the electrode 5 is bent to the upstream side, namely, in the direction opposite to the direction of flow of the exhaust gas, in this embodiment, the electrode 5 may be bent to the downstream side, namely, in the direction of flow of the exhaust gas. If the electrode 5 is bent toward the upstream side of the flow of exhaust gas as in this embodiment, the PM is less likely or unlikely to be deposited on the insulator portion 51. Namely, the PM can be charged at the upstream side of the insulator portion 51, and the charged PM is directed onto the inner circumferential surface of the housing 3. Therefore, the amount of PM that collides with the insulator portion 51 is reduced, and thus the PM is less likely or unlikely to be deposited on the insulator portion 51. However, if the electrode 5 is bent toward the upstream side of the flow of exhaust gas, the electrode 5 is more likely to be deformed when receiving force from the flow of the exhaust gas. Therefore, the arrangement of this embodiment is suitable for the case where the electrode 5 is short. On the other hand, if the electrode 5 is bent toward the downstream side of the flow of exhaust gas, the PM is more likely to be deposited on the insulator portion 51, but the electrode 5 is less likely or unlikely to be deformed even if it receives force from the flow of the exhaust gas. In this case, therefore, the durability and reliability of the electrode 5 can be enhanced, and the length of the electrode 5 can be increased.

The controller 7 is configured to change the voltage applied to the electrode 5, and determine whether the particulate matter treatment system 1 is at fault, based on the amount of change in the number of PM particles detected by the particle number sensor 75 before and after the change of the applied voltage. In this embodiment, an absolute value of the amount of change is used as a failure criterion.

If the particulate matter treatment system 1 is normal, electrons are released from the electrode 5 when a voltage is applied to the electrode 5, resulting in aggregation of the PM particles. As the applied voltage becomes larger, a larger number of electrons are released from the electrode 5, and therefore, the amount of aggregated particles in the PM increases. Namely, if the particulate matter treatment system 1 is normal, the amount of change in the number of PM particles when the applied voltage is changed becomes relatively large.

If, on the other hand, the particular matter treatment system 1 is at fault, no electrons may be released from the electrode 5, or the amount of release of electrons may not be sufficiently large. As a result, the amount of aggregated PM particles is reduced, and therefore, the amount of change in the number of PM particles when the applied voltage is changed becomes relatively small. For example, when a failure or problem that no voltage is applied to the electrode 5 occurs, the applied voltage does not change but remains zero even if an attempt to change the applied voltage is made. Therefore, the number of PM particles detected by the detector 9 is equal to the same value before and after the applied voltage is changed. Namely, the amount of change in the number of PM particles is equal to zero.

Accordingly, if a threshold value is set for the amount of change in the number of PM particles when the applied voltage is changed, it can be determined that the particulate matter treatment system 1 is at fault when the detected amount of change is smaller than the threshold value. The threshold value is determined in advance by experiment, or the like, as a lower limit of the amount of change in the number of PM particles when the particulate matter treatment system 1 is normal.

FIG. 2 is a flowchart illustrating the control flow of failure determination according to this embodiment. The routine of FIG. 2 is repeatedly executed by the controller 7 at given time intervals.

In step S101, operating conditions of the internal combustion engine are acquired. For example, values, such as those of the engine speed, engine load, and the temperature of the internal combustion engine, which will be required for subsequent processing are read. The engine speed is detected by the crank position sensor 72, and the engine load is detected by the acceleration stroke sensor 71. The temperature of the engine (e.g., the temperature of the lubricating oil or the temperature of the coolant) is detected by the temperature sensor 73.

In step S102, a voltage applied to the electrode 5 is calculated. The applied voltage is set in accordance with the estimated number of PM particles (per cm³), for example. The number of PM particles is the number of PM particles discharged from the internal combustion engine, which number is measured before the exhaust gas flows into the housing 3. The number of PM particles is correlated with the engine speed, engine load, and the temperature of the internal combustion engine (e.g., the temperature of the lubricating oil or the temperature of the coolant), and is therefore calculated based on these values. A plurality of maps for calculating the number of PM particles, from the engine speed and the engine load, may be stored in relation to the temperature of the engine, and the number of PM particles may be calculated based on a selected one of the maps.

A sensor that detects the number of PM particles may be mounted in the exhaust passage 2 upstream of the housing 3, and the number of PM particles may be detected by the sensor. Also, a detection value of the particle number sensor 75 obtained when no voltage is applied to the electrode 5 may be used.

Then, the applied voltage is calculated based on the number of PM particles and the amount (g/sec.) of exhaust gas of the internal combustion engine. This relationship may be obtained in advance by experiment, or the like, and may be mapped. The amount of exhaust gas of the engine is correlated with the amount of intake air of the engine, and can be thus obtained based on the intake air amount detected by the airflow meter 74.

As the amount of exhaust gas is smaller, the inertial force of the PM is reduced, and therefore, an influence of electrostatic actions is relatively increased. As a result, the PM particles are more likely to aggregate. Accordingly, as the amount of exhaust gas is smaller, the PM particles aggregate with a smaller voltage applied to the electrode 5. Therefore, the applied voltage is reduced as the amount of exhaust gas is smaller. As the number of PM particles is larger, the distance between the PM particles becomes shorter, and therefore, an influence of electrostatic actions is relatively increased. Therefore, as the number of PM particles is larger, the PM particles aggregate with a smaller voltage applied to the electrode 5. Thus, the applied voltage is reduced as the number of PM particles is larger.

The applied voltage may also be set to a value at which the percentage of reduction of the number of PM particles becomes equal to a give value (e.g., 40%). The percentage of reduction of the number of PM particles is the ratio of the number of PM particles reduced in the housing 3 to the number of PM particles flowing into the housing 3. Also, the applied voltage may be set to a predetermined, specified value. After the applied voltage is calculated in this manner, the voltage is applied to the electrode 5, and the control goes to step S103.

In step S103, it is determined whether it is the time for making a failure decision. For example, the determination of step S103 is made by determining whether the accumulated value of the operating time of the internal combustion engine as measured from the time when a failure decision was made the last time reaches a predetermined value. Namely, a failure decision is made each time the accumulated value of the operating time reaches the predetermined value. For example, it may be determined that it is the time for making a failure decision when the accumulated operating time of the engine is a multiple of the predetermined value. Also, it may be determined that it is the time for making a failure decision when the travel or running distance of the vehicle on which the engine is installed is equal to a multiple of a predetermined value. The above-indicated predetermined values are set in advance as values at which a failure decision is required to be made. If an affirmative decision (YES) is made in step S103, the control goes to step S104. If a negative decision (NO) is made in step S103, the routine of FIG. 2 ends.

In step S104, it is determined whether the internal combustion engine is in steady operation. The number of PM particles varies depending on the operating conditions of the engine; therefore, it may be difficult to make a failure decision if the operating conditions of the engine change while a failure decision is being made. Accordingly, in this embodiment, a failure decision is made during steady operation of the engine. It is determined that the engine is in steady operation, when the amounts of change of the engine speed obtained by the crank position sensor 72 and the engine load obtained by the acceleration stroke sensor 71 over a given period of time are within predetermined ranges in which the engine can be said to be in a steady state. If an affirmative decision (YES) is made in step S104, the control goes to step S105. If a negative decision (NO) is made in step S104, the routine of FIG. 2 ends.

In step S105, the number of PM particles detected by the particle number sensor 75 is obtained. In this step, the number of PM particles before the applied voltage is changed is detected. The number of PM particles obtained in this step is denoted as “pre-voltage-change number of particles PM1”.

In step S106, the applied voltage is changed. For example, the applied voltage is reduced by a predetermined value. The applied voltage may also be set to zero. As a result, the PM is less likely to aggregate, and therefore, the number of PM particles detected by the detector 9 increases. The applied voltage may be increased, so that the PM is more likely to aggregate, and the number of PM particles is reduced.

In step S107, the number of PM particles detected by the particle number sensor 75 is obtained. In this step, the number of PM particles after the applied voltage is changed is detected. The number of PM particles obtained in this step is denoted as “post-voltage-change number of particles PM2”.

In step S108, the amount of change in the number of PM particles is calculated. This amount is expressed as an absolute value. Namely, the amount of change is calculated according to the following equation (1).

Amount of Change=|PM2−PM1|  (1)

In step S109, it is determined whether the amount of change in the number of PM particles calculated in step S108 is smaller than a threshold value. In this step, it is determined whether the number of PM particles is sufficiently changed due to the change of the applied voltage. The threshold value is a lower limit of the amount of change in the number of PM particles when the particulate matter treatment system 1 is normal, and is obtained in advance by experiment, or the like. If an affirmative decision (YES) is made in step S109, the control goes to step S110. If a negative decision (NO) is made in step S109, the routine of FIG. 2 ends since no failure is found in the particulate matter treatment system 1. In this embodiment, the controller 7 that executes step S109 corresponding to the determining device of the invention.

In step S110, a failure flag is set to ON. The failure flag is set to ON when the particulate matter treatment system 1 is at fault, and is set to OFF when the system 1 is not at fault. The initial value of the failure flag is OFF. If the failure flag is set to ON, a warning light is turned on so as to inform the driver of the vehicle of the presence of a failure.

As explained above, according to this embodiment, a failure of the particulate matter treatment system 1 can be determined, based on the amount of change in the number of PM particles calculated from the numbers of PM particles before and after the applied voltage is changed. Also, the current or applied voltage need not be detected when a failure decision is made. It is possible to enhance the accuracy of the failure determination by changing the applied voltage two or more times, and calculating the amount of change in the number of PM particles two or more times. It is also possible to enhance the accuracy of the failure determination by calculating the amount of change in the number of PM particles at each applied voltage when the applied voltage is reduced or increased in steps.

Next, a second embodiment of the invention will be described. In the second embodiment, the threshold value used in step S109 of FIG. 2 is changed based on at least one of the number of PM particles and the amount of exhaust gas of the internal combustion engine. In the second embodiment, the devices, etc. other than the controller 7 are identical with those of the first embodiment, and therefore, will not be explained.

Initially, the case where the threshold value is changed according to the number of PM particles (the number of particles detected by the particle number detector) will be explained.

The threshold value may be changed according to the number of particles detected by the particle number detector when no voltage is applied from the power supply to the electrode. Also, the threshold value may be changed according to the number of particles of particulate matter discharged from the internal combustion engine. The number of particles of the particulate matter discharged from the engine may be detected by a sensor, or may be estimated based on the operating conditions of the engine. As the number of PM particles is larger, the distance between the PM particles becomes shorter, and an influence of electrostatic actions is relatively increased. Therefore, as the number of PM particles is larger, the PM particles aggregate at a smaller applied voltage. Accordingly, as the number of PM particles is larger, an absolute value of the amount of change in the number of particles when the applied voltage is changed is increased. Thus, the threshold value can be changed in accordance with the number of PM particles. Namely, since the PM particles are more likely to aggregate as the number of PM particles is larger, the absolute value of the amount of change in the number of PM particles should become larger if the particulate matter treatment system is normal. Therefore, the threshold value can be increased as the number of PM particles is larger, and can be reduced as the number of PM particles is smaller. Consequently, the accuracy with which a failure decision is made can be further enhanced.

The above-described situation will be described in greater detail with reference to FIG. 3. FIG. 3 is a graph indicating the relationships among the applied voltage, the number of PM particles, and the percentage of reduction of the number of PM particles. FIG. 3 shows four patterns of relationships (indicated by circles, triangles, diamonds, and inverted triangles) involving different numbers of PM particles flowing into the housing 3. The applied voltage is equal to 0 (kV) when application of voltage to the electrode 5 is stopped. Namely, the number of PM particles when the applied voltage is 0 (kV) corresponds to the number of PM particles flowing into the housing 3. The number of PM particles flowing into the housing 3 is largest in the case of the circles indicated in FIG. 3, and becomes smaller in the order of the triangles, diamonds, and inverted triangles as indicated in FIG. 3.

In the case of the circles in which the number of PM particles flowing into the housing 3 is largest, the percentage of reduction of the number of PM particles reaches the highest level among the four patterns indicated in FIG. 3. On the other hand, in the case of the inverted triangles in which the number of PM particles flowing into the housing 3 is smallest, the percentage of reduction of the number of PM particles is lowest among the four patterns. It is thus understood from FIG. 3 that the percentage of reduction of the number of PM particles increases as the number of PM particles flowing into the housing 3 is larger. It is also understood that the amount of change in the number of PM particles when the applied voltage is changed is larger as the number of PM particles flowing into the housing 3 is larger.

As the number of PM particles is larger, the distance between the PM particles becomes shorter, and an influence of electrostatic actions is relatively increased. Therefore, as the number of PM particles is larger, the PM particles aggregate at a smaller applied voltage. Thus, if the particulate matter treatment system 1 is normal, the amount of change in the number of PM particles when the applied voltage is changed increases as the number of PM particles flowing into the housing 3 is larger. The number of PM particles can vary due to individual differences among internal combustion engines and/or chronological changes of the internal combustion engine.

Accordingly, the accuracy of the failure determination can be enhanced by changing the threshold value according to the number of PM particles. If the particulate matter treatment system 1 is normal, the amount of change in the number of PM particles when the applied voltage is changed should increase as the number of PM particles is larger. Therefore, the threshold value is increased as the number of PM particles is larger. For example, the relationship between the number of PM particles and a correction factor is obtained in advance by experiment, or the like. Then, the correction factor is obtained from the detected or estimated number of PM particles upstream of the housing 3, and the threshold value is changed by multiplying the threshold value by the correction factor.

Thus, the accuracy of the failure determination can be enhanced by increasing the threshold value as the number of PM particles is larger.

The amount of change in the number of PM particles when the applied voltage is changed also varies depending on the amount of exhaust gas of the internal combustion engine.

Thus, an exhaust gas amount detector for detecting or estimating the amount of exhaust gas emitted from the engine may be further provided, and the threshold value may be changed according to the exhaust gas amount detected by the exhaust gas amount detector.

As the exhaust gas amount is smaller, the inertial force of the PM is reduced, and an influence of electrostatic actions is relatively increased. Therefore, the PM particles are more likely to aggregate. Accordingly, as the exhaust gas amount is smaller, the PM particles aggregate at a smaller applied voltage. Therefore, as the exhaust gas amount is smaller, (the absolute value of) the amount of change in the number of PM particles when the applied voltage is changed increases. Thus, the threshold value can be changed in accordance with the exhaust gas amount. Namely, since the PM particles are more likely to aggregate as the exhaust gas amount is smaller, the absolute value of the amount of change in the number of PM particles should become larger if the particulate matter treatment system is normal. Thus, the threshold value can be increased as the exhaust gas amount is smaller, and can be reduced as the exhaust gas amount is larger. Consequently, the accuracy of the failure determination can be further enhanced.

The above-described situation will be described in greater detail with reference to FIG. 4. FIG. 4 is a graph indicating the relationships among the applied voltage, the number of PM particles, and the percentage of reduction of the number of PM particles. FIG. 4 shows four patterns of relationships (indicated by circles, triangles, diamonds, and inverted triangles) involving different amounts of exhaust gas emitted from the internal combustion engine. The exhaust gas amount is largest in the case of the circles indicated in FIG. 4, and becomes smaller in the order of the triangles, diamonds, and inverted triangles. The applied voltage is equal to 0 (kV) when application of voltage to the electrode 5 is stopped. The operating conditions of the engine are controlled so that the number of PM particles when the applied voltage is 0 (kV) is equal to substantially the same value no matter how large the exhaust gas amount is.

In the case of the circles in which the exhaust gas amount is largest, the percentage of reduction of the number of PM particles is lowest among the four patterns indicated in FIG. 4. On the other hand, in the case of the inverted triangles in which the exhaust gas amount is smallest, the percentage of reduction of the number of PM particles is highest among the four patterns. It is thus understood from FIG. 4 that as the exhaust gas amount is smaller, the amount of change in the number of PM particles when the applied voltage is changed becomes larger.

Accordingly, the accuracy of failure detection can be enhanced by changing the threshold value according to the exhaust gas amount. As the exhaust gas amount is smaller, the amount of change in the number of PM particles when the applied voltage is changed should become larger if the particulate matter treatment system 1 is normal. Therefore, the threshold value is increased as the exhaust gas amount is smaller. For example, the relationship between the exhaust gas amount and the correction factor is obtained in advance by experiment, or the like. Then, a correction factor is obtained from the detected exhaust gas amount, and the threshold value is changed by multiplying the threshold value by the obtained correction factor. Since the exhaust gas amount (g/sec) of the internal combustion engine is correlated with the intake air amount of the engine, the exhaust gas amount may be obtained based on the intake air amount detected by the airflow meter 74. In this embodiment, the controller 7 that calculates the exhaust gas amount corresponds to the above-indicated exhaust gas amount detector of the invention.

Thus, the accuracy of failure determination can be further enhanced by increasing the threshold value as the exhaust gas amount is smaller.

Furthermore, the threshold value may be changed based on both the number of PM particles and the exhaust gas amount. For example, the relationships among the number of PM particles, the exhaust gas amount, and the correction factor are obtained in advance by experiment, or the like. Then, a correction factor is obtained from the detected number of PM particles and exhaust gas amount, and the threshold value is changed by multiplying the threshold value by the obtained correction factor. Consequently, the accuracy of failure detection can be further enhanced.

According to the embodiment as described above, the threshold value is changed based on at least one of the number of PM particles and the exhaust gas amount of the internal combustion engine, so that the accuracy of failure detection can be further enhanced. 

1. A particulate matter treatment system, comprising: an electrode provided in an exhaust passage of an internal combustion engine; a power supply connected to the electrode and operable to apply a voltage to the electrode; a particle number detector that detects the number of particles of particulate matter downstream of the electrode; and a determining device that determines that the particulate matter treatment system is at fault when an absolute value of an amount of change in the number of particles of particulate matter detected by the particle number detector when the voltage applied from the power supply to the electrode is changed is smaller than a threshold value.
 2. The particulate matter treatment system according to claim 1, wherein the threshold value is changed according to the number of particles detected by the particle number detector.
 3. The particulate matter treatment system according to claim 2, wherein the threshold value is set to a larger value as the number of particles detected by the particle number detector is larger.
 4. The particulate matter treatment system according to claim 1, further comprising an exhaust gas amount detector that detects or estimates an amount of exhaust gas emitted from the internal combustion engine, wherein the threshold value is changed according to the amount of exhaust gas detected by the exhaust gas amount detector.
 5. The particulate matter treatment system according to claim 4, wherein the threshold value is set to a larger value as the amount of exhaust gas detected by the exhaust gas amount detector is smaller.
 6. The particulate matter treatment system according to claim 2, further comprising an exhaust gas amount detector that detects or estimates an amount of exhaust gas emitted from the internal combustion engine, wherein the threshold value is changed according to the amount of exhaust gas detected by the exhaust gas amount detector.
 7. The particulate matter treatment system according to claim 6, wherein the threshold value is set to a larger value as the amount of exhaust gas detected by the exhaust gas amount detector is smaller. 